Confused about black levels... again.

[binister]

I wrestled with black levels during my VHS->DVD conversion process I went through earlier this year. I thought I had things down until I just tried to process some video from my new Flip MinoHD video recorder.

The Flip is very simple and I am going to use it as my daily carry but I was surprised when I looked at the waveform of the recorded video in VDub.

Here is a screen cap...
http://img296.imageshack.us/img296/6258/blackleveluj9.jpg

The black levels are recorded below level 16... why is this? I thought all digital video recorders record between digital 16-235.

The other strange thing is when I view this video in QuickTime or VLC the black levels are all different. QuickTime is the lightest, VLC medium and VDub is the darkest.

Any help would be appreciated.

[jagabo]

The black levels are recorded below level 16... why is this? I thought all digital video recorders record between digital 16-235.

They are supposed to be between 16 and 235. In practice you often see stuff outside that range. Since you are using AviSynth and VideoScope() I would guess the levels you see are what's in the source (unless you did any colorspace conversions incorrectly before VideoScope()). If you upload a few seconds of the source (unprocessed) I can verify this for you.

The other strange thing is when I view this video in QuickTime or VLC the black levels are all different. QuickTime is the lightest, VLC medium and VDub is the darkest.

The difference is in how they do the YUV to RGB conversion and what output device they are using.

Quicktime converts to RGB and uses Windows GDI for output. It sounds like it is not performing the usual YUV to RGB contrast stretch. The equivalent of ConvertToRGB(matrix="PC.709") or ConvertToRGB(matrix="PC.601") in AviSynth.

VLC will most likely leave the video in YUV space and use the graphics card's video overlay feature (depending on options chosen). The output will be effected by the overlay proc amp settings.

VirtualDub (if fed YUV from AviSynth) will perform the usual contrast stretch when converting to RGB. This is the equivalent of ConvertToRGB() or ConvertToRGB(matrix="rec601").

[binister]

Just to follow up on this thread... It appears my conversion from MP4 to AVI was the issue. I was using MPEG StreamClip which uses QuickTime to do the conversion. From what I have gathered, QuickTime on the PC suffers from a luma conversion bug when using hardware acceleration.

When I view the waveform of the MP4 when opened in VDub using DirectShow() and converted to YUY the black levels only drop to digital 16.

I then realized that VDub can DirectStream copy to AVI which I then put through CCE for MPEG2 encoding.

This stuff gets complicated sometimes! Thanks for the help.

[edDV]

The Flip illustrates the reason camcorders should use 16-235. Note that all the whites on the dog are hard clipped at 255. The white detail exceeded 255 and was discarded. There is nothing you can do to recover that detail. If the camera was using 16-235, the overshoots in the 236-255 region could be scaled back to 235 in post with all detail present.

The only workaround you have may be reduced exposure in high contrast situations or use lights to raise the black level.

[jagabo]

Everything about Quicktime is screwed up in Windows.

I found an unprocessed MP4 sample from the Flip MinoHD in some review yesterday. Its black levels looked OK. The whites were blown out at times though. The blow out was probably caused by the auto exposure not compensating quickly enough as the camera panned across a bright area (only a half dozen frames or so).

[vhelp]

what really frustrates me at times is that even after obtaining source, you can never truely know with certainty where the sources levels were to begin with. So, its hard at times to arrive at the correct levels. That, plus, then there is the final destination video format and playback aspects to consider.

Even on television broadcasts, the levels on some stations and/or program content can differ. Some can be on account of "washout" look, and others from purposed-treated reasons. Some shows are dark, while others are lighter, and so on and so forth. Some broadcasts or video content (ie, dvd, vhs, laserdisc, etc) are either done properly or not, and we've seen this in many instances here..or at least, mentioned.

So even if you can't get it right, consider the possibility that you have a source that was prob not correct to begin with or was swaded because of the codec (decoder) being used on your given system.

-vhelp 4964

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